Photovoltaic system and grid connection method of photovoltaic system

Abstract

The invention is suitable for the technical field of power electronics, and provides a photovoltaic system and a grid-connected method of the photovoltaic system, and the photovoltaic system comprises a photovoltaic string unit, an inverter, a grid-connected switch and a frame circuit breaker. The photovoltaic string unit is connected with the input end of the inverter; the output end of the inverter is connected with the input end of the grid-connected switch; the output end of the grid-connected switch is connected with the input end of the frame circuit breaker; the output end of the frame circuit breaker is used for being connected with a three-phase power grid. The grid-connected switch comprises a plurality of power switch tubes; the frame circuit breaker is used for closing under the condition that the inverter responds to a grid-connected instruction; and the plurality of power switch tubes are used for conducting under the condition that the electric signal output by the inverter is consistent with the electric signal of the three-phase power grid, so as to complete the closing of the grid-connected switch. According to the photovoltaic system and the grid connection method of the photovoltaic system provided by the invention, rapid grid connection is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and in particular relates to a photovoltaic system and a grid-connected method for the photovoltaic system. Background Art

[0002] In the related art, the photovoltaic system adopts a centralized inverter and uses the following two solutions to achieve grid connection.

[0003] like Figure 1 As shown, the first solution is that the photovoltaic system is equipped with an AC contactor and a frame circuit breaker. The frame circuit breaker is only used as a disconnecting device. When the inverter is fully started until the grid-connected conditions are met, a command to close the AC contactor is issued. After the AC contactor is closed, the grid is successfully connected.

[0004] like Figure 2 As shown, the second solution is that the photovoltaic system has a frame circuit breaker with a long count life. The frame circuit breaker not only serves as a disconnecting device but also assumes the function of a grid-connected switch. When the inverter is fully started until the grid-connected conditions are met, a command to close the frame circuit breaker is issued, and the grid is successfully connected after the frame circuit breaker is closed.

[0005] The second solution is cheaper than the first solution and saves an AC contactor, but both solutions have the problem of long grid-connected switch closing process time. More and more requirements in the industry are that the inverter should be quickly powered on and connected to the grid after receiving the grid dispatch command, and the shorter the grid-connected time, the better. The speed of grid-connected time directly affects the stability and reliability of the power system. If the grid-connected time is too long, it may cause instability of the power system and even cause grid failure. Summary of the invention

[0006] The embodiments of the present invention provide a photovoltaic system and a method for connecting the photovoltaic system to the grid, aiming to solve the problem that a long time for connecting the photovoltaic system to the grid may cause instability of the power system and even cause grid failure.

[0007] The present invention provides a photovoltaic system, comprising: a photovoltaic string unit, an inverter, a grid-connected switch and a frame circuit breaker;

[0008] The photovoltaic string unit is connected to the input end of the inverter; the output end of the inverter is connected to the input end of the grid-connected switch; the output end of the grid-connected switch is connected to the input end of the frame circuit breaker; the output end of the frame circuit breaker is used to connect to the three-phase power grid;

[0009] The grid-connected switch includes a plurality of power switch tubes;

[0010] The frame circuit breaker is used to close when the inverter responds to a grid connection instruction;

[0011] The plurality of power switch tubes are used to be turned on when the electrical signal output by the inverter is consistent with the electrical signal of the three-phase power grid, so as to complete the closing of the grid-connected switch.

[0012] In some embodiments, each phase output end of the inverter is connected to at least one power switch tube.

[0013] In some embodiments, when there are multiple at least one power switch tubes, the at least one power switch tube is connected in series.

[0014] In some embodiments, when there are multiple at least one power switch tubes, the at least one power switch tube is connected in parallel.

[0015] In some embodiments, when the at least one power switch tube is an odd number, the at least one power switch tube is divided into multiple groups of power switch tubes, each group of power switch tubes includes one or more power switch tubes, and the multiple groups of power switch tubes are connected in parallel.

[0016] In some embodiments, when the at least one power switch tube is an even number, the at least one power switch tube is divided into multiple groups of power switch tubes, the number of power switch tubes in each group is the same and the power switch tubes included in each group of power switch tubes are connected in series, and the multiple groups of power switch tubes are connected in parallel.

[0017] In some embodiments, the plurality of groups of power switch tubes are an even number of groups, and every two groups of power switch tubes are connected in reverse parallel.

[0018] In some embodiments, the power switch tube is an IGBT.

[0019] The present invention further provides a photovoltaic system grid-connected method, which is applied to the photovoltaic system as described above, and the method comprises:

[0020] Receive grid connection instructions;

[0021] In response to the grid connection instruction, a first closing instruction is issued to the frame circuit breaker, and an electrical signal output by the inverter is determined;

[0022] When the electrical signal output by the inverter is consistent with the electrical signal of the three-phase power grid, a second closing instruction is issued to the grid-connected switch, wherein the second closing instruction is used to instruct the plurality of power switch tubes to be turned on;

[0023] When the frame circuit breaker and the grid-connected switch are both closed and the inverter outputs a grid-connected current, it is determined that the grid-connected operation is successful.

[0024] In some embodiments, the time when the plurality of power switch tubes complete conduction is earlier than the time when the frame circuit breaker completes closing.

[0025] The photovoltaic system and the grid-connected method of the photovoltaic system provided by the present invention adopt a plurality of power switch tubes as grid-connected switches, which are connected in series with the frame circuit breaker to combine the fast closing time of the power switch, the high breaking capacity of the frame circuit breaker and the ability of the frame circuit breaker to physically isolate the power grid. The frame circuit breaker is closed when the inverter responds to the grid-connected instruction, and the grid-connected switch is closed when the inverter reaches the grid-connected conditions, thereby forming a fast grid-connected system, which can realize the fast grid-connected function of the inverter and meet the demand for fast dispatching of the inverter. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is one of the structural schematic diagrams of the photovoltaic system provided by the prior art;

[0027] Figure 2 This is the second structural schematic diagram of the photovoltaic system provided by the prior art;

[0028] Figure 3 It is a grid-connected timing diagram of a photovoltaic system provided by the prior art;

[0029] Figure 4 It is one of the structural schematic diagrams of the photovoltaic system provided by the embodiment of the present invention;

[0030] Figure 5 This is one of the structural schematic diagrams of the grid-connected switch of the photovoltaic system provided by the embodiment of the present invention;

[0031] Figure 6 This is the second structural schematic diagram of the grid-connected switch of the photovoltaic system provided by the embodiment of the present invention;

[0032] Figure 7 is a schematic diagram of a grid-connected process of a photovoltaic system provided by an embodiment of the present invention;

[0033] Figure 8 This is the second structural schematic diagram of the photovoltaic system provided by the embodiment of the present invention;

[0034] Fig. 9 It is a grid-connected timing diagram of a photovoltaic system provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0036] Reference to "embodiments" or "implementations" in the present invention means that the specific features, components, or characteristics described in conjunction with the embodiments or implementations may be included in at least one embodiment of the present invention. The appearance of the phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0037] In the description of the present invention, it should be understood that the terms "length", "width", "up", "down", "left", "right", "horizontal", "top", "bottom" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first" and "second" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, the features defined as "first" and "second" may explicitly or implicitly include one or more of the said features. In the description of the present invention, the meaning of "multiple" is two or more, unless otherwise clearly and specifically defined.

[0038] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, an electrical connection, or mutual communication; it can be a direct connection, or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0039] In related technologies, with the severe cost problem in the photovoltaic industry, most suppliers reduce costs by increasing the power level of the inverter or the system voltage level. Therefore, the power of the inverter in the photovoltaic system is getting bigger and bigger, and the system voltage level is increased from the original 1000V system to the 1500V system and is moving towards a system above 2000V. At present, high-voltage photovoltaic systems mostly use centralized inverters, and costs are further reduced by increasing the power level of the centralized inverter or increasing the system voltage level.

[0040] Figure 1 and Figure 2The AC contactors or frame circuit breakers used in the system are all mechanical switches, and the closing time of mechanical switches is discrete, generally around 30ms to 60ms. Adding the AC soft start time of the inverter, which is at least 20ms, the total grid connection time exceeds 70ms. However, the current requirement is that the inverter must complete the grid connection process within 50ms of the grid dispatch command.

[0041] For the grid-connected process of the solution in the related art, the grid-connected timing diagram is as follows Figure 3 As shown. At t0, the inverter receives the power-on command from the power station, that is, the grid-connected command. The inverter performs AC soft start, and the electrical signal output by the inverter starts to track the electrical signal of the power grid from zero. At t1, the electrical signal output by the inverter is consistent with the electrical signal of the power grid, meeting the grid-connected conditions, and issuing a closing command to the frame circuit breaker. The frame circuit breaker is a mechanical switch. After receiving the closing command from t1, it takes about 50ms for the frame circuit breaker to complete the closing action. That is, at t2, the inverter outputs the grid-connected current, and the inverter completes the grid-connected work. From the time the power station issues the command to the time the inverter completes the grid-connected work, the time to connect the grid-connected current to the grid is from t0 to t2, which takes about 70ms, and the grid-connected time is relatively long.

[0042] Combine the following Figure 4-Figure 9 The photovoltaic system and the method for connecting the photovoltaic system to the grid provided by the present invention are described.

[0043] Figure 4 Schematic diagram of the structure of the photovoltaic system provided by the embodiment of the present invention. Figure 4 The photovoltaic system provided by the present invention comprises:

[0044] Photovoltaic string unit 10, inverter 20, grid-connected switch 30 and frame circuit breaker 40;

[0045] The photovoltaic string unit 10 is connected to the input end of the inverter 20; the output end of the inverter 20 is connected to the input end of the grid-connected switch 30; the output end of the grid-connected switch 30 is connected to the input end of the frame circuit breaker 40; the output end of the frame circuit breaker 40 is used to connect to the three-phase power grid 50;

[0046] The grid-connected switch 30 includes a plurality of power switch tubes;

[0047] The frame circuit breaker 40 is used to close when the inverter 20 responds to the grid connection command;

[0048] A plurality of power switch tubes are used to be turned on when the electrical signal output by the inverter 20 is consistent with the electrical signal of the three-phase power grid 50 , so as to complete the closing of the grid-connected switch 30 .

[0049] In actual implementation, the photovoltaic string unit 10 is a minimum unit with a required DC output voltage formed by connecting multiple photovoltaic modules in series. The number of photovoltaic modules in the photovoltaic string unit 10 depends on the system design requirements and is not specifically limited here.

[0050] The inverter 20 is mainly used to convert the direct current output by the photovoltaic string unit 10 into alternating current so as to be connected to the three-phase power grid 50 or used by local loads.

[0051] The grid-connected switch 30 can realize the regulation of the three-phase grid 50 by disconnecting or connecting the circuit, thereby ensuring the safe operation of the three-phase grid 50. The grid-connected switch 30 uses a plurality of power switch tubes with high voltage resistance, low conduction loss and fast switching speed, which is helpful to realize fast grid connection.

[0052] The power switch tube includes, for example, an insulated gate bipolar transistor IGBT, a metal oxide semiconductor field effect transistor MOSFET, a thyristor or a power triode.

[0053] The frame circuit breaker 40 is mainly used as a disconnecting device in the photovoltaic system. It has high disconnecting capacity and high reliability, and can quickly cut off the circuit in the event of a short circuit or overload to protect the safety of the equipment and system.

[0054] In actual implementation, the inverter 20 in the photovoltaic system receives the grid connection command issued by the power station, and at the same time issues the closing instruction of the frame circuit breaker 40, and then the inverter 20 performs AC soft start. When the electrical signal output by the inverter 20 is consistent with the electrical signal of the three-phase power grid 50, the closing instruction of the grid connection switch is issued. The conduction time of the power switch tube used in the grid connection switch 30 is very fast, so the grid connection switch 30 can be closed within microseconds.

[0055] It should be noted that the electrical signal output by the inverter 20 is consistent with the electrical signal of the three-phase grid 50, which is a basic requirement for the grid-connected operation of the inverter 20. The consistency of the electrical signal involves multiple parameters, including voltage, frequency, phase, etc., which together determine whether the inverter 20 can be smoothly and efficiently connected to the grid.

[0056] 1. Voltage consistency

[0057] The voltage output by the inverter 20 needs to be consistent with the voltage of the three-phase grid 50 in amplitude and waveform. Specifically, the voltage amplitude output by the inverter 20 should be equal to or slightly higher than the voltage of the three-phase grid 50 to ensure that there is no impact on the three-phase grid 50 during the grid connection process. At the same time, the voltage waveform output by the inverter 20 should be close to a sine wave to reduce the interference of harmonics on the three-phase grid 50. In practical applications, the inverter 20 is usually equipped with a voltage regulation function, which can automatically adjust the output voltage according to the change of the voltage of the three-phase grid 50 to maintain the consistency of the voltage.

[0058] 2. Frequency consistency

[0059] The frequency of the AC power output by the inverter 20 needs to be consistent with the frequency of the three-phase grid 50. In most areas, the frequency of the three-phase grid 50 is 50 Hz or 60 Hz, so the frequency of the AC power output by the inverter 20 should also match this. Frequency consistency is crucial for the synchronous operation and stability of the power system. If the frequency output by the inverter 20 is inconsistent with the frequency of the three-phase grid 50, it will cause the frequency of the three-phase grid 50 to fluctuate, thereby affecting the stability and power quality of the three-phase grid 50.

[0060] Phase consistency

[0061] The phase of the AC power output by the inverter 20 needs to be consistent with the voltage phase of the three-phase grid 50. Phase consistency is the key to achieving grid-connected operation of the inverter 20. If the phase of the inverter 20 output is inconsistent with the voltage phase of the three-phase grid 50, it will cause current shock and power factor reduction during grid connection. Therefore, the inverter 20 usually uses a phase-locked loop (PLL) technology to track the phase of the three-phase grid 50 to ensure that the phase of the AC power output by the inverter 20 is consistent with the grid voltage phase.

[0062] 4. Harmonic Suppression

[0063] The inverter 20 may generate harmonics in the process of converting DC power into AC power, and these harmonics may cause interference and loss to the three-phase power grid 50. Therefore, the inverter 20 needs to meet certain harmonic limitation standards to reduce the impact of harmonics on the three-phase power grid 50. In practical applications, the inverter 20 is usually equipped with a filter or modulation strategy to suppress the generation of harmonics.

[0064] In summary, the electrical signal output by the inverter 20 is consistent with the electrical signal of the three-phase grid 50, and needs to meet the consistency requirements of multiple parameters such as voltage, frequency and phase. At the same time, the inverter 20 also needs to adopt a suitable grid-connected control strategy to ensure the stability and efficiency of grid-connected operation.

[0065] It can be understood that when the frame circuit breaker 40 and the grid-connected switch 30 are both closed and the inverter 20 outputs the grid-connected current, it is determined that the grid-connection is successful.

[0066] The photovoltaic system provided by the present invention adopts a plurality of power switch tubes as grid-connected switches, which are connected in series with the frame circuit breaker to combine the fast closing time of the power switch, the high breaking capacity of the frame circuit breaker and the ability of the frame circuit breaker to physically isolate the power grid. The frame circuit breaker is closed when the inverter responds to the grid-connected instruction, and the grid-connected switch is closed when the inverter reaches the grid-connected conditions, thereby forming a fast grid-connected system, which can realize the fast grid-connected function of the inverter and meet the demand for fast dispatching of the inverter.

[0067] In some embodiments, each phase output terminal of the inverter 20 is connected to at least one power switch tube.

[0068] In actual implementation, the grid-connected switch 30 has a three-phase input terminal and a three-phase output terminal, each phase input terminal is electrically connected to the inverter 20, and each phase output terminal is electrically connected to the frame circuit breaker 40. The grid-connected switch 30 includes a plurality of power switch tubes, and at least one power switch tube is arranged corresponding to each phase, so that the input terminal of at least one power switch tube of each corresponding phase is electrically connected to the inverter 20, and the output terminal of at least one power switch tube of each corresponding phase is electrically connected to the frame circuit breaker 40. Among them, the plurality of power switch tubes is greater than or equal to 3. The number of power switch tubes arranged in each phase can be set according to actual needs and is not specifically limited here. In the embodiment of the present invention, increasing the number of power switch tubes can improve the reliability and stability of the photovoltaic system and reduce the possibility of failure.

[0069] Among them, the power switch tube is a semiconductor device that can control the current. Its working principle is to control the conduction and cutoff of the power switch tube by controlling the voltage change. When the control voltage of the power switch tube is high, the power switch tube is in the on state and the current can flow; when the control voltage is low, the power switch tube is in the off state and the current cannot pass.

[0070] In some embodiments, when there are multiple at least one power switch tubes, at least one power switch tube is connected in series.

[0071] In actual implementation, when there is one corresponding at least one power switch tube, the power switch tube is directly connected in series between the inverter 20 and the frame circuit breaker 40 .

[0072] When there are two or more corresponding at least one power switch tube, at least one power switch tube is fully connected in series. The fully connected power switch tubes can form a whole and be connected in series between the inverter 20 and the frame circuit breaker 40 .

[0073] In some embodiments, when there are multiple at least one power switch tubes, at least one power switch tube is connected in parallel.

[0074] In actual implementation, when there are two corresponding at least one power switch tubes, the two power switch tubes are connected in parallel, wherein the parallel connection may include forward parallel connection or reverse parallel connection.

[0075] Forward parallel connection refers to directly connecting the corresponding pins of two power switch tubes together and then connecting them to the circuit. Make sure that the same pins of each power switch tube are connected together to achieve forward parallel connection.

[0076] Reverse series connection refers to connecting the current input end of one power switch tube to the current output end of another power switch tube, and connecting the current output end of one power switch tube to the current input end of another power switch tube to achieve reverse parallel connection.

[0077] For example: the three electrodes of a field effect transistor are source, gate and drain, the source of the field effect transistor is the current input terminal, and the drain is the current output terminal; or the three electrodes of a bipolar transistor are emitter, base and collector, the emitter of the bipolar transistor is the current input terminal, and the collector of the bipolar transistor is the current output terminal. The three electrodes of an insulated gate bipolar transistor are emitter, gate and collector, the emitter of the insulated gate bipolar transistor is the current input terminal, and the collector of the insulated gate bipolar transistor is the current output terminal.

[0078] When there are more than two power switch tubes in each corresponding phase, the multiple power switch tubes are connected in parallel. The number of power switch tubes included in each phase can be the same or different, which is not specifically limited here.

[0079] In some embodiments, when at least one power switch tube is an odd number, the at least one power switch tube is divided into multiple groups of power switch tubes, each group of power switch tubes includes one or more power switch tubes, and the multiple groups of power switch tubes are connected in parallel.

[0080] In actual implementation, at least one power switch tube can be 3, 5, 7...2N+1, where N is a positive integer. At least one power switch tube is divided into 2 or more groups of power switch tubes, each group of power switch tubes includes one or more power switch tubes, at least two groups of power switch tubes have different numbers, and each group of power switch tubes is connected in parallel.

[0081] For example: if there are three power switch tubes, the at least one power switch tube can be divided into three groups, each group corresponds to one power switch tube, and the three power switch tubes are connected in parallel; if there are three power switch tubes, the at least one power switch tube can be divided into two groups, the first group corresponds to one power switch tube, the second group corresponds to two power switch tubes, and the two groups of power switch tubes are connected in parallel;

[0082] If there are at least five power switch tubes, at least one power switch tube can be divided into two groups, the first group can correspond to two power switch tubes, and the second group can correspond to three power switch tubes, or at least one power switch tube can be divided into three groups, the first group can correspond to two power switch tubes, the second group can correspond to two power switch tubes, and the third group can correspond to two power switch tubes; or at least one power switch tube can be divided into four groups, the first group can correspond to two power switch tubes, the second group can correspond to one power switch tube, the third group can correspond to one power switch tube, and the fourth group can correspond to one power switch tube; no matter which grouping is selected, each group of power switch tubes is connected in parallel.

[0083] In some embodiments, when at least one power switch tube is an even number, at least one power switch tube is divided into multiple groups of power switch tubes, the number of power switch tubes in each group is the same and the power switch tubes included in each group of power switch tubes are connected in series, and the multiple groups of power switch tubes are connected in parallel.

[0084] In actual implementation, the number of power switch tubes may be 4, 6, 8, ... 2N, where N is a positive integer. The power switch tubes are divided into multiple groups of power switch tubes, each group of power switch tubes includes at least one power switch tube, and the power switch tubes in each group are connected in parallel.

[0085] For example: if there are 4 power switch tubes, at least one power switch tube can be divided into 2 groups, each group corresponds to 2 power switch tubes, and the 2 groups of power switch tubes are connected in parallel; if there are 6 power switch tubes, at least one power switch tube can be divided into 3 groups, each group corresponds to 2 power switch tubes, and the 3 groups of power switch tubes are connected in parallel; or at least one power switch tube can be divided into 2 groups, each group corresponds to 3 power switch tubes, and the 2 groups of power switch tubes are connected in parallel; no matter which grouping is selected, each group of power switch tubes is connected in parallel.

[0086] In some embodiments, the plurality of groups of power switch tubes are an even number of groups, and every two groups of power switch tubes are connected in reverse parallel.

[0087] In actual implementation, at least one power switch tube can be divided into 2 groups, 4 groups, 6 groups...2N groups of power switch tubes, N is a positive integer, and the number of power switch tubes contained in each group is the same and the power switch tubes included in each group of power switch tubes are connected in series.

[0088] In this embodiment, every two groups of power switch tubes are connected in reverse parallel. In every two groups of power switch tubes, the current input end of the first group of power switch tubes is connected to the current output end of the second group of power switch tubes, and the current output end of the first group of power switch tubes is connected to the current input end of the second group of power switch tubes.

[0089] Taking the power switch tube as IGBT as an example, the power switch tube connected to one phase is explained.

[0090] like Figure 5 As shown, there are 2 corresponding power switch tubes, the 2 power switch tubes are divided into 2 groups, and the 2 power switch tubes are connected in reverse parallel, that is, the emitter of the first IGBT of the 2 IGBTs is connected to the collector of the second IGBT, and the collector of the first IGBT is connected to the emitter of the second IGBT, thereby realizing reverse parallel connection.

[0091] like Figure 6 As shown, each corresponding number of power switch tubes is 4, and the 4 power switch tubes are divided into 2 groups, each group includes 2 power switch tubes connected in forward series, and the 2 groups of power switch tubes are connected in reverse parallel. The emitter of the first group of IGBTs in the 2 groups of IGBTs is connected to the collector of the second group of IGBTs, and the collector of the first group of IGBTs is connected to the emitter of the second group of IGBTs, thereby realizing reverse parallel connection.

[0092] In some embodiments, the power switch tube is an IGBT.

[0093] In actual implementation, the closing time of IGBT is very fast and can complete the grid-connected action within microseconds.

[0094] Figure 7 Schematic diagram of the process of the photovoltaic system grid connection method provided by the embodiment of the present invention. Figure 7 The present invention provides a method for connecting a photovoltaic system to the grid, which is applied to the photovoltaic system in any of the above embodiments, and the method comprises:

[0095] Step 110: receiving a grid connection instruction;

[0096] Step 120: respond to the grid connection instruction, issue a first closing instruction to the frame circuit breaker, and determine the electrical signal output by the inverter;

[0097] Step 130: When the electrical signal output by the inverter is consistent with the electrical signal of the three-phase power grid, a second closing instruction is issued to the grid-connected switch, where the second closing instruction is used to instruct a plurality of power switch tubes to be turned on;

[0098] Step 140: When the frame circuit breaker and the grid-connected switch are both closed and the inverter outputs the grid-connected current, it is determined that the grid-connected is successful.

[0099] In actual implementation, the inverter in the photovoltaic system receives a grid-connected instruction, which refers to a power-on command sent by the power station to the inverter of the photovoltaic system to instruct the inverter to start running.

[0100] The inverter receives the grid connection instruction from the power station, and the inverter starts slowly. When the inverter receives the grid connection instruction, it sends the first closing instruction to the frame circuit breaker, the frame circuit breaker starts to close, and the inverter performs AC soft start.

[0101] The voltage output by the inverter increases from small to large, and the relevant inverter algorithm is used to start the AC inverter voltage to track the grid from zero until the electrical signal output by the inverter is consistent with the electrical signal of the three-phase grid, meeting the grid connection requirements, and then the second closing instruction is issued to the grid-connected switch to turn on several power switch tubes. Several power switch tubes are voltage-type devices and can complete the closing process within a few microseconds. When the frame circuit breaker and the grid-connected switch are closed and the inverter outputs the grid-connected current, it is determined that the grid connection is successful, thereby completing the grid connection.

[0102] The grid-connected method for a photovoltaic system provided by the present invention adopts a plurality of power switch tubes as grid-connected switches, which are connected in series with a frame circuit breaker, thereby combining the fast closing time of the power switch, the high breaking capacity of the frame circuit breaker and the ability of the frame circuit breaker to physically isolate the power grid, thereby forming a fast grid-connected system, which can realize the fast grid-connected function of the inverter and meet the demand for fast dispatching of the inverter.

[0103] In some embodiments, the time when the plurality of power switch tubes complete conduction is earlier than the time when the frame circuit breaker completes closing.

[0104] In actual implementation, the moment when the frame breaker is driven to close is the same as the moment when the inverter starts tracking the grid.

[0105] It is understandable that the frame circuit breaker is a mechanical switch, and the time to complete the closing action must be greater than the time for the power switch tube to complete the conduction. Therefore, the time when several power switch tubes complete the conduction is earlier than the time when the frame circuit breaker completes the closing.

[0106] The embodiments of the present invention are described below in conjunction with actual application scenarios.

[0107] like Figure 8As shown, the photovoltaic system includes photovoltaic string units, DC load switches, inverters, inverter inductors, grid-connected switches, frame circuit breakers, and transformers connected in sequence. The AC filter capacitor is connected in parallel to the output end of the inverter inductor, and the output end of the transformer is connected to a 35KV high-voltage three-phase grid.

[0108] The photovoltaic system uses frame circuit breakers as breaking devices. The frame circuit breakers have high breaking capacity and the ability to physically isolate the power grid. Several IGBT modules are used as grid-connected switches. The IGBT modules are voltage-type devices that can complete the closing process within a few microseconds and have the ability to close quickly.

[0109] like Fig. 9 As shown, the grid-connected process of the photovoltaic system can be explained by the grid-connected timing diagram.

[0110] At t0, the inverter in the photovoltaic system receives the power-on command from the power station and sends a closing command to the frame circuit breaker. The frame circuit breaker is a mechanical switch. It takes about 50ms to complete the closing action after receiving the closing command at t0. The inverter starts at t0, and the electrical signal output by the inverter starts to track the grid from zero. It takes at least 20ms for the voltage to increase from small to large. At t1, the electrical signal output by the inverter is consistent with the electrical signal of the grid, meeting the grid connection requirements, and sending several closing commands to the IGBT module.

[0111] The IGBT module is a voltage-type device that can complete the closing process within a few microseconds. At t2, the IGBT module is already turned on, and the grid-connected switch is closed. At this time, the electrical signal output by the inverter is consistent with the electrical signal of the power grid. Until t3, the frame circuit breaker is also closed, the inverter completes the grid-connected work, and the inverter outputs the grid-connected current, so that the photovoltaic system is successfully connected to the grid. In other words, the time from the power station issuing the grid-connected command to the inverter completing the grid-connected work and connecting the grid-connected current to the grid is from t0 to t3, which takes about 50ms. Since the IGBT only takes a few microseconds to turn on, most of the 50ms time is the closing action time of the frame circuit breaker, and the inverter's grid-connected time is much shorter than that of traditional solutions.

[0112] The grid-connected method for a photovoltaic system provided by the present invention adopts a method of connecting a frame circuit breaker and an IGBT module in series, combining the fast closing time of the IGBT module, the high breaking capacity of the frame circuit breaker and the ability to physically isolate the power grid to form a fast grid-connected system that can meet the current market requirements for fast grid-connected inverters.

[0113] It is understandable that those skilled in the art can, under the guidance of the above embodiments, combine various implementation methods in the above embodiments to obtain technical solutions of multiple implementation methods.

[0114] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A photovoltaic system, characterized in that: include: PV string units, inverters, grid-connected switches and frame circuit breakers; The photovoltaic string unit is connected to the input end of the inverter; the output end of the inverter is connected to the input end of the grid-connected switch; the output end of the grid-connected switch is connected to the input end of the frame circuit breaker; the output end of the frame circuit breaker is used to connect to the three-phase power grid; The grid-connected switch includes a plurality of power switch tubes; The frame circuit breaker is used to close when the inverter responds to a grid connection instruction; The plurality of power switch tubes are used to be turned on when the electrical signal output by the inverter is consistent with the electrical signal of the three-phase power grid, so as to complete the closing of the grid-connected switch.

2. The photovoltaic system according to claim 1, characterized in that: Each phase output end of the inverter is connected to at least one power switch tube.

3. The photovoltaic system according to claim 2, characterized in that: In the case that there are multiple at least one power switch tubes, the at least one power switch tube is connected in series.

4. The photovoltaic system according to claim 2, characterized in that: In the case that there are multiple at least one power switch tubes, the at least one power switch tube is connected in parallel.

5. The photovoltaic system according to claim 4, characterized in that: When the at least one power switch tube is an odd number, the at least one power switch tube is divided into a plurality of groups of power switch tubes, each group of power switch tubes includes one or more power switch tubes, and the plurality of groups of power switch tubes are connected in parallel.

6. The photovoltaic system according to claim 4, characterized in that: When the at least one power switch tube is an even number, the at least one power switch tube is divided into multiple groups of power switch tubes, the number of power switch tubes in each group is the same and the power switch tubes included in each group of power switch tubes are connected in series, and the multiple groups of power switch tubes are connected in parallel.

7. The photovoltaic system according to claim 6, characterized in that: The multiple groups of power switch tubes are an even number group, and every two groups of power switch tubes are reversely connected in parallel.

8. The photovoltaic system according to any one of claims 1 to 7, characterized in that: The power switch tube is an IGBT.

9. A method for connecting a photovoltaic system to a grid, applied to the photovoltaic system according to any one of claims 1 to 7, characterized in that: include: Receive grid connection instructions; In response to the grid connection instruction, a first closing instruction is issued to the frame circuit breaker, and an electrical signal output by the inverter is determined; When the electrical signal output by the inverter is consistent with the electrical signal of the three-phase power grid, a second closing instruction is issued to the grid-connected switch, wherein the second closing instruction is used to instruct the plurality of power switch tubes to be turned on; When the frame circuit breaker and the grid-connected switch are both closed and the inverter outputs a grid-connected current, it is determined that the grid-connected operation is successful.

10. The method for connecting a photovoltaic system to the grid according to claim 9, characterized in that: The time when the plurality of power switch tubes complete conduction is earlier than the time when the frame circuit breaker completes closing.

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